Control system with filtered dead zone

1. A method of operating a control device having a set point value, an error signal, and a control output signal, the error signal representative of a difference between a process variable and the set point value, the control output signal operable to affect the process variable, the method comprising:

a) generating the control output signal based at least in part on the error signal;

b) filtering the error signal to generate a filtered error signal; and using the filtered error signal to determine whether a dead zone operation comprising generating the control output signal independent of the error signal should occur.

2. The method of claim 1 wherein step c) further comprises holding the control output signal as a held value independent of the error signal when the filtered error signal is within the dead zone.

3. The method of claim 1 wherein step b) further comprises filtering the error signal with a low pass filter.

4. The method of claim 1 wherein step b) further comprises filtering the error signal with a low pass filter having a time constant that is based in part on a time constant of an integrating portion of the control device.

5. The method of claim 1 wherein step b) further comprises filtering the error signal with a digital filter.

6. The method of claim 1 further comprising:

d) resuming the generation of the control output signal based at least in part on the error signal when the filtered error signal passes outside of the dead zone.

7. The method of claim 1 wherein step c) further comprises holding the control output signal at a held value independent of the error signal when the filtered error signal falls below a first threshold, and further comprising step d) resuming the generation of the control output signal base at least in part on the error signal when the filtered error signal exceeds a second threshold, the second threshold greater than the first threshold.

8. The method of claim 7 wherein the second threshold is based at least in part on the set point value.

9. A control system comprising:

an actuator having a first input, the actuator operable to effect a process in response to the first input; and

a controller operably coupled to receive a process variable and a set point value, the process variable representative of an output of the process, the controller further operably connected to provide a control output signal to the first input of the actuator, the controller operable to

generate the control output signal based at least in part on the error signal;

filter the error signal to generate a filtered error signal;

use the filtered error signal to determine whether a dead zone operation comprising generating the control output signal independent of the error signal should occur.

10. The control system of claim 9 wherein the controller is further operable to hold the control output signal as a held value independent of the error signal when the filtered error signal is within the dead zone.

11. The control system of claim 9 wherein the controller is further operable to filter the error signal with a low pass filter.

12. The control system of claim 9 wherein the controller is further operable to filter the error signal with a low pass filter having a time constant that is based in part on a time constant of an integrating portion of the control device.

13. The control system of claim 9 wherein the controller is further operable to filter the error signal with a digital filter.

14. The control system of claim 9 wherein the controller is further operable to resume the generation of the control output signal based at least in part on the error signal when the filtered error signal passes outside of the dead zone.

15. The control system of claim 9 wherein the controller is further operable to hold the control output signal at a held value independent of the error signal when the filtered error signal falls below a first threshold, and to resume the generation of the control output signal base at least in part on the error signal when the filtered error signal exceeds a second threshold, the second threshold greater than the first threshold.

16. The control system of claim 15 wherein the second threshold is based at least in part on the set point value.